The heater does use whatever is cooling the block. If you have raw water cooling then the heater needs to be drained. You should be able to blow the lines clear. It's hard to get antifreeze in them all the way to the core.

I don't use antifreeze and I have never had an issue- just get the water out. Frankly, if you take the hose off the block that connects to the heater, you can blow it out easily without compressed air. Just make sure you get all of the water out otherwise when you fire it up in the spring you'll have a flood under your dash from cracking the coil.

not to sound dumb, but guess i'm just plain ole ignorant... but how do you know all the water is out... i've never had a boat w/ a heater til this one... what if the heater core is lower than the lines...

Tickle, you will get a rush of water from the return line once you put compressed air through and you'll be able to tell when its just air. It won't hurt to blow air back the opposite direction just for good measure.

I remove the input line to the heater from the intake on the engine and blow air through it after I have drained the block. Leave the block drains open and you will see the water from the heater core drain. You can fill the block/heater core with RV antifreeze through the heater hose fitting on the intake. Never had a problem doing it this way.

i messed w/ it yesterday. removed the line from the intake and the line from the circ-pump (aggravating) blew compressed air through the line removed from the intake and a large amount of water came out the other line... 2 questions.

i gather i can leave the line at the pump connected and water will come out the eng block.??? if i pour antifreeze in the hose from the intake port, how on earth will it make it to the heater core. the heater lines run along the bilge (in essence the sag between intake and heater core..?? how can 90 psi damage a heater core? --it's not direct psi...

I've always blown mine out with a compressor..never had any problems. Depending on how the heater hoses are routed it will fill the heater core from the bottom to the top if the hose from the intake is left off while filling and you fill through the intake. The fluid level should work similar to a short piece of hose bent in a "u shape/sag" fill one side and it will level off between each side of the hose until it fills and over flows. Put a short piece of hose on the intake fitting and fill through the intake. I'm not an expert, but it's worked pretty well for me the last few years.